This invention is in the field of control circuitry for magnetic storage media drives, and is more specifically directed to fly height control circuitry involving read/write head heaters in such drives.
Continuing progress toward higher performance yet less expensive electronic systems has resulted in large part from advances in nonvolatile data storage technology. The “workhorse” technology for nonvolatile data storage has, of course, been the magnetic disk drive. Magnetic disk drives are used over a wide range of electronic systems, including large-scale network servers, desktop workstations, portable computers, and even now in modern handheld devices such as portable digital audio players.
As is well known in the art, the capacity of conventional disk drives has greatly increased over recent years, at ever decreasing cost per megabit. This capacity increase is directly related to improvements in the density with which data can be stored on the disk medium. Advances in disk drive technology have decreased the surface area required to reliably and retrievably store a bit along a “track” on the disk surface, and have also decreased the spacing between adjacent tracks. This reduction in the active disk surface area per unit of storage has been enabled, in large part, by corresponding reductions in the size and precision of the magnetic transducers that effect the writing and reading operations in magnetic disk drives.
In conventional magnetic disk drives, the writing and reading of stored data is carried out by way of near-field magnetic processes. To write data, ferromagnetic domains at the disk surface are selectively oriented by applying a magnetic field in close proximity to the disk surface. One type of conventional write transducer, or “head”, is the well-known inductive writer, which includes an electromagnet having a gap that can be positioned near the magnetic disk surface. The electromagnet is selectively energized to establish a magnetic field, at the gap, that is strong enough to define a magnetic transition pattern of the desired polarity at the addressed location of the disk surface. Data is read from the disk by sensing the polarity of the magnetic field established by these magnetic transition patterns. Conventional read transducers include inductive heads consisting of an electromagnet (which may be the same electromagnet used to write data) in which a current is induced by the magnetic fields at the disk surface. More recently, magnetoresistive (MR) read heads, having a resistance that varies with the polarity of the magnetic field, have become popular.
In modern disk drives, the read/write heads are disposed within a slider at the distal end of a head gimbal assembly (HGA) suspension. The flexible HGA suspension is attached to an actuator, which includes a so-called “voice coil” motor that positions the heads at the desired radial locations of the disk surface. The relative motion between the spinning disk surface and the slider creates a lifting force on the slider, establishing an air bearing surface (ABS) on which the slider rides over the disk surface. Typically, the heads are located at the trailing edge of the slider, which is typically closer to the disk surface than is the slider leading edge.
In connection with these near-field mechanisms, the magnetic field strength increases exponentially as the distance between the magnetic transducers (read/write heads) and the magnetized disk surface decreases. Because the signal strength of the stored data depends on the magnetic field strength, the density of data storage per unit area of disk surface, for a given bit error rate (BER), depends strongly on the distance between the heads and the disk surface. Accordingly, the spacing maintained by the air bearing surface (ABS) between the read/write heads and the disk surface, referred to in the art as the “fly height” of the heads, is an important parameter in the data capacity of a magnetic disk. In modern conventional disk drives, the mean fly height is on the order of a few nanometers, which has enabled the very high data densities attained by modern disk drives.
However, low fly heights tend to increase the wear of both the disk surface and the read-write heads. At extremely low fly heights, relatively small asperities in the disk surface can cause contact between the slider and the disk surface, depleting and degrading lubricants, causing wear on both the slider and the disk surface and causing contamination from wear particles, and in some cases causing the heads to stick at locations of the disk surface where contact is made.
Disk drive manufacturers are thus faced with a tradeoff between disk drive reliability, on one hand, and data density and BER, on the other hand, in determining the desired fly height of the read/write heads. The precision with which the fly height can be controlled is therefore an important factor in maximizing the data density and minimizing the BER, within the reliability constraints for the disk drive.
Especially in recent years, the fly height has depended largely on the temperature of the poles of the inductive write head. It is known that the writing current conducted by the inductive write head causes resistive heating and thus thermal expansion of the poles in the read/write head, typically manifest in the electromagnet poles protruding from the slider toward the disk surface. Modern disk drives take advantage of the thermal expansion of the write head poles in achieving low fly heights, and in controlling the fly height. In this regard, it is known to include a resistor within the slider of a disk drive read/write head, to serve as a resistive heater, so that thermal expansion of the electromagnet poles cause them to protrude toward the disk surface, reducing the fly height. U.S. Pat. No. 5,991,113 describes an example of such a resistive heater.
By way of further background, copending and commonly assigned U.S. patent application Ser. No. 10/715,217, filed Nov. 17, 2003, and published as U.S. Patent Application Publication No. US 2005/0105204 on May 19, 2005, describes a fly height controller that applies different drive levels to a resistive heater in read and write operations. These different heater drive levels account for the increased current drive to the inductive write heads, and thus increased heating of the slider, during write operations relative to the current conducted through the head during read operations.
Conventional fly height controllers using a resistive heater in the read/write head assembly operate by feedback control of either the voltage across the resistive heater, or of the current through the resistive heater. For previous generations of disk drives, control of the resistive heater drive in this manner has been sufficient. As the desired data density continues to increase, however, and with the need for continuing miniaturization of disk drives, more precise fly height control has become necessary.